There is nothing to stop you from fitting a bathtub within an ensuite bathroom, provided there is adequate space to accommodate it, and allow free movement in and around the tub without impacting on other fixtures and fittings.
That said, the vast majority of ensuite bathroom areas are unlikely to be big enough to house a freestanding bathtub in particular, and will instead feature shower cubicles, for instance.
For more information and tips regarding key fittings for en-suite bathrooms, take a look at our blog, Excellence In Ensuite Bathroom Ideas.
